1. Field of the Invention
The invention relates to a formulation which comprises at least one silane and at least one carbon polymer, and to the production of a silicon layer on a substrate which is coated with such a formulation.
2. Description of the Related Art
The conventional production of solar cells consists either in the opposite doping of a doped semiconductor substrate by means of implantation or diffusion, in the deposition of an oppositely doped semiconductor layer on a doped semiconductor substrate by means of epitaxy, or in the deposition of semiconductor layers of different doping from the gas phase under reduced pressure. The processes mentioned are also employed in variants.
The disadvantage of established processes is the level of complexity which is necessary owing to the need for vacuum technology, high temperatures and/or substrates prepared at great cost and inconvenience. To avoid this complexity, cost and inconvenience, attempts are being made to produce semiconductive or photovoltaically active layers or layer sequences from liquid silanes.
Silanes can be deposited from the liquid phase thereof by means of a spin-coater. The layers obtained therefrom can be stabilized by a suitable thermal treatment, such that they typically comprise a mixture of microcrystalline, nanocrystalline and amorphous structures. Such microcrystalline, nanocrystalline and/or amorphous layers are referred to here and hereinafter as ‘polymorphic’. An exact distinction and definition of the crystalline structure of polymorphic layers is not possible with accuracy in most cases, or is of minor importance for the intended applications.
The way in which silicon layers are produced from silanes is known to those skilled in the art. For instance, GB 2077710 teaches the preparation of polysilanes of the general formula —(SiH2),— where n≧10 by simultaneous reduction and polymerization of SiH2Cl2 with alkali metals.
Such higher silanes are used as precursors for silicon layers, for example for solar cells. In the case of the silanes SinH2+2 with relatively small values for n, namely n≦4, JP 7267621 teaches the production of silicon layers from films of such silanes, which are first irradiated with UV light under cold conditions and then heated to temperatures above 400° C. EP 1 284 306 discloses that silicon films can be produced in a similar manner from cyclic silanes of the general formula SinH2n, and open-chain silanes of the general formula SinH2n+2, each where 3≦n≦10.
These silanes are partly or fully oligomerized, for example by heating and/or UV irradiation. In addition, specific phosphorus compounds or boron compounds are added in order to achieve n- or p-doping.
The above-described production of such layers and the layers themselves have considerable defects. Firstly, the liquid silane compounds, alone or in formulated solution, have an extremely low polarity. The result of this is that the spread of a film composed of such formulations in the coating operation is poor and the films contract after the shear by the application unit has ended. This makes the coating outcome fully or partly reversible even at the wet film stage, and the result is incompletely covered substrates, for example with a leopardskin structure consisting of covered and uncovered regions. It is possible to counteract these film defects to a small degree by reducing the silane content in the formulation and/or by higher shear, for example high speed of the spin-coater. However, such measures lead to thin films, and it is barely possible to exceed film thicknesses of 100 nm.
A certain improvement in film formation and a reduction in the contraction effect of the films is also achieved by the oligomerization of the liquid silanes or by dissolution of silane oligomers in the liquid formulations. However, these measures are limited by the fact that oligomeric silanes of relatively high molar masses are insoluble in the low molecular weight silanes and in the solvents added thereto and, for example according to EP 1 284 306, only about 10 percent by weight of oligomers are soluble in the liquid silane formulation. Since the silane oligomers have a low polarity and must not have a very high molecular weight in order still to dissolve in the low molecular weight silane, only minor improvements can be achieved in this way. The trend towards incompletely covered substrates is therefore maintained.
Relatively high layer thicknesses of 1 to 3 μm are advantageous in particular for the lower layer of a solar cell, known as the base layer. Thicker layers of liquid silanes are achieved by means of relatively high-viscosity formulations and/or the buildup of multiple layers. In the case of buildup of multiple layers by repeated coating, however, the problem of inadequate coating of the substrate remains. Although there is a certain probability of covering previously uncovered regions by repeated coating steps, the multiple layers then achieved have significant thickness variations. Moreover, these are not reproducible owing to random results of such processes.
On silicon as the substrate, liquid silane-based silicon layers are achievable without any problems. However, it is not possible according to the prior art to obtain a first coherent silicon layer on a suitable substrate in order then to be able to add on the further layers more easily. It is therefore not possible to produce solar cells on the basis of a sequence of thin polymorphic silicon layers by means of spin-on deposition or a similar process. It is thus not possible to achieve a layer structure which is capable of absorbing sufficient sunlight.
The procedure outlined above also does not allow the layer to be provided with a structure. Structuring of the layer is desirable since light absorption can be increased not only by virtue of the layer thickness itself, but also by virtue of structuring of the silicon layers. Light can be refracted many times by edges and recesses, which are equivalent to refractive index contrasts, which leads to lengthening of the light path in the photovoltaically active medium. There is thus effectively more light available for the photovoltaic operation. Structuring can be achieved in conventional wafer or CVD processes, for example by roughening the layer surfaces by means of etching processes, but are not achievable in a simple manner on liquid silane-based silicon layers.
In summary, it can be stated that it has not been possible to date in the prior art to obtain, from spread liquid silanes, silicon layers which completely cover the substrate, are structurable like the layers obtained from conventional wafer or CVD processes, and in addition have thicknesses which are already usable for photovoltaic applications when the liquid silanes are applied only once to the substrate.